Pet system

ABSTRACT

A PET apparatus ( 1 ) includes a detecting section ( 10 ). Each cylindrical detector ( 13   n  ) of the detecting section ( 10 ) has a plurality of block detectors ( 14   1  to  14   M ) arranged on the same circumference on a place perpendicular to a central axis (CAX) in the form of a ring. Each block detector ( 14   m ) is a two-dimensional position detector which detects the two-dimensional incident position of a photon incident on a light-receiving surface  15   b . Each slice collimator ( 21   n ) extends to a rear portion of a corresponding one of cylindrical detectors ( 13   n ) through the space between adjacent cylindrical detectors ( 13   n ) and ( 13   n+1 ) and is integrally fixed by a holding plate ( 22 ) at the rear portion.

TECHNICAL FIELD

[0001] The present invention relates to a PET apparatus which can imagethe behavior of a substance marked by a positron emission isotope (RIradiation source).

BACKGROUND ART

[0002] A PET (Positron Emission Tomography) apparatus is an apparatuswhich can image the behavior of a trace substance in an object (livingbody) to be examined by detecting a pair of 511 keV photons (gamma rays)which fly in opposite directions upon electron-positron pairannihilation in the object irradiated with RI radiation. The PETapparatus includes a detecting section having many small photondetectors arrayed around a measurement space in which an object to beexamined is placed. This apparatus detects a photon pair generated uponelectron-positron pair annihilation by coincidence counting, accumulatesthe coincidence counting information, and reconstructs an imagerepresenting the spatial distribution of occurrence frequencies ofphoton pairs in the measurement space on the basis of these many piecesof accumulated coincidence counting information. This PET apparatusserves an important role in the field of nuclear medicine and the like.For example, biofunctions and the high-order brain functions can bestudied by using this apparatus. Such PET apparatuses are roughlyclassified into two-dimensional PET apparatuses and three-dimensionalPET apparatuses.

[0003]FIG. 7 is a view for explaining the arrangement of the detectingsection of a two-dimensional PET apparatus. FIG. 7 shows an example ofan arrangement including seven detector rings, and is a sectional viewof the detecting section taken along a plane including the central axis.A detecting section 10 of the two-dimensional PET apparatus has detectorrings R₁ to R₇ stacked between a shield collimator 11 and a shieldcollimator 12. Each of the detector rings R₁ to R₇ has a plurality ofphoton detectors arranged in the form of a ring on a plane perpendicularto the central axis. Each photon detector is a scintillation detectorformed from a combination of a scintillator such as BGO (Bi₄Ge₃O₁₂) anda photomultiplier. This detector detects photons flying from ameasurement space including the central axis. The two-dimensional PETapparatus has slice collimators S₁ to S₆ inside the detecting section10. These slice collimators S₁ to S₆ are ring-like members each of whichis placed between adjacent detector rings in a direction parallel to thecentral axis. Each slice collimator is made of a material having alarger atomic number and larger specific gravity (e.g., lead ortungsten) and has a collimating function of shielding obliquely incidentphotons (gamma rays).

[0004] The detecting section 10 of the two-dimensional PET apparatushaving the above arrangement can perform coincidence counting of only aphoton pair flying from the nearly 90° direction with respect to thecentral axis owing to the collimating function of the slice collimatorsS₁ to S₆. That is, the coincidence counting information, i.e.,two-dimensional projection data, accumulated by the detecting section 10of the two-dimensional PET apparatus is limited to that obtained by apair of photon detectors included in a single detector ring or detectorrings which are adjacent to each other (or very close to each other).The two-dimensional PET apparatus can therefore efficiently removescattered radiation produced when a photon pair generated outside themeasurement space is scattered. In addition, this apparatus can easilyperform absorption correction and sensitivity correction with respect totwo-dimensional projection data, and hence can obtain a reconstructedimage with good quantitativeness.

[0005]FIG. 8 is a view for explaining the arrangement of the detectingsection of the three-dimensional PET apparatus. FIG. 8 is also asectional view of the detecting section taken along a plane includingthe central axis. The arrangement of the detecting section 10 of thethree-dimensional PET apparatus is the same as that of thetwo-dimensional PET apparatus except that the three-dimensional PETapparatus has no slice collimators. The detecting section 10 of thethree-dimensional PET apparatus having this arrangement has a wide solidangle and can perform coincidence counting of a photon pair flying froma wide range as compared with the two-dimensional PET apparatus. Thatis, as the coincidence counting information, i.e., three-dimensionalprojection data, obtained and accumulated by the detecting section 10 ofthe three-dimensional PET apparatus, data obtained by a pair of photondetectors included in an arbitrary detector ring can be used.Three-dimensional PET apparatus can therefore perform coincidencecounting of a photon pair with sensitivity five to ten times higher thanthat of the two-dimensional PET apparatus. As compared with thetwo-dimensional PET apparatus, however, the three-dimensional PETapparatus has difficulty in accurately removing the influence ofscattered radiation, and hence the quantitativeness of a reconstructedimage is poor.

[0006] As described above, as compared with the three-dimensional PETapparatus, the two-dimensional PET apparatus having slice collimatorshas low photon pair detection sensitivity but can efficiently removescattered radiation and easily perform absorption correction andsensitivity correction. The two-dimensional PET apparatus therefore hasthe merit of obtaining a reconstructed image with excellentquantitativeness.

DISCLOSURE OF INVENTION

[0007] However, the present inventor has found the following problems inthe above two-dimensional PET apparatus. In the two-dimensional PETapparatus, the slice collimators S₁ to S₆ exist only inside thedetecting section 10 (between the measurement space and the detectingsection 10) and are fixed to each other inside the detecting section 10by a holding plate. In the two-dimensional PET apparatus, it isimportant to ensure the relative positional relationship betweendetector rings R_(n) and slice collimators S_(n) of the detectingsection 10 such that they are alternately arranged in a directionparallel to the central axis. If the precision of the relativepositional relationship between the detector rings R_(n) and the slicecollimators S_(n) is poor, the slice collimator S_(n) may be located infront of the light-receiving surface of the detector ring R_(n),resulting in a decrease in the incidence efficiency of photons on eachdetector ring R_(n). This causes a deterioration in the performance ofthe two-dimensional PET apparatus. In order to ensure this relativepositional precision, process accuracy and assembly accuracy for therespective detector rings R_(n), the respective slice collimators S_(n),and the like must be strictly managed. This makes it difficult tomanufacture this apparatus. This leads to an increase in the cost of thetwo-dimensional PET apparatus.

[0008] In addition, in the two-dimensional PET apparatus, since theholding plate for fixing the slice collimators S₁ to S₆ to each other isplaced inside the detecting section 10, photons generated in themeasurement space are absorbed by the holding plate, resulting in adecrease in the photon detection sensitivity of the detecting section10. In general, the holding plate is required to have high mechanicalstrength to hold the respective slice collimators S_(n) made of amaterial having a larger atomic number and larger specific gravity. Theholding plate is also required to absorb a small amount of photons. Forthis reason, for example, a high-strength resin such as a carbon fiberresin or an aluminum alloy is used for the holding plate. In addition,this plate needs to have a thickness of about 6 mm to 10 mm inconsideration of a relationship with strength. When aluminum exhibitinga radiation attenuation coefficient of 0.2269/cm with respect to photons(gamma rays) is used for the holding plate, photons are absorbed by theholding plate by about 10% to 20%. As described above, in thetwo-dimensional PET apparatus, photons generated in the measurementspace are absorbed by the holding plate, and hence the photon detectionsensitivity of the detecting section 10 decreases.

[0009] In the two-dimensional PET apparatus, since the holding plate forfixing the slice collimators S₁ to S₆ to each other exists inside thedetecting section 10, there is a limit in brining the respectivedetector rings R_(n) close to the respective slice collimators S_(n),and a predetermined distance is required between them in the radialdirection. For this reason, although a photon passing through betweenthe slice collimator S_(n−1) and the slice collimator S_(n) shouldstrike the detector ring R_(n), it may strike the adjacent detector ringR_(n−1) or R_(n+1). This decreases the reliability of accumulatedcoincidence counting information and affects the quality of areconstructed image.

[0010] In addition, the present inventor has found that the above two-and three-dimensional PET apparatuses have the following problems. Boththe two- and three-dimensional PET apparatuses are required to improvethe resolution of a reconstructed image. In order to improve theresolution, it is dispensable to reduce the size of each photondetector.

[0011] In the case of the two-dimensional PET apparatus, however, as thesize of each photon detector decreases, the intervals between therespective slice collimators decrease, resulting in a decrease in openarea ratio. This causes a deterioration in photon pair detectionsensitivity. With regard to this problem, in the two-dimensional PETapparatus, a decrease in open area ratio can be suppressed by thinningand shortening each slice collimator in accordance with a reduction inthe size of each photon detector. However, the effect of shieldingphotons (gamma rays), i.e., the collimating effect, deteriorates. Thismakes it impossible to efficiently remove scattered radiation, resultingin a deterioration in the quantitativeness of a reconstructed image.

[0012] In the case of the three-dimensional PET apparatus, even if thesize of each photon detector is reduced, since no slice collimator isused, a reduction in open area ratio and a deterioration in photon pairdetection sensitivity do not occur. As described above, however, in thethree-dimensional PET apparatus, since it is essentially difficult toremove the influence of scattered radiation, the quantitativeness of areconstructed image is poor.

[0013] It is an object of the present invention to provide a PETapparatus which can improve photon detection sensitivity.

[0014] A PET apparatus according to one aspect of the present inventionis characterized by comprising (1) a detecting section which includes aplurality of cylindrical detectors each formed by one- ortwo-dimensionally arraying a plurality of photon detection elements,each of which detects a photon flying from a measurement space includinga central axis, on a cylinder surrounding the central axis, theplurality of cylindrical detectors being arrayed in a direction parallelto the central axis, (2) a plurality of slice collimators which arealternately arranged with the cylindrical detectors in a directionparallel to the central axis, each of the slice collimators extendingfrom a position between the measurement space and the detecting sectionto a rear portion of a corresponding one of the cylindrical detectorsthrough a space between two adjacent cylindrical detectors of theplurality of cylindrical detectors and passing only a photon, of photonsflying from the measurement space, which is substantially parallel to apredetermined plane perpendicular to the central axis toward thedetecting section, (3) a coincidence counting information accumulatingsection which accumulates coincidence counting information of a photonpair detected by one pair of photon detection elements included in thedetecting section, and (4) an image reconstructing section whichreconstructs an image representing a spatial distribution of occurrencefrequencies of photon pairs in the measurement space on the basis of thecoincidence counting information accumulated by the coincidence countinginformation accumulating section.

[0015] In the PET apparatus according to the above aspect of the presentinvention, a photon pair, of photon pairs generated in the measurementspace, which has reached the detecting section without being shielded bythe slice collimator is simultaneously detected by one pair of photondetection elements included in the detecting section, and thecoincidence counting information is converted into information in acoordinate system fixed to the object and accumulated by the coincidencecounting information accumulating section. When the accumulation ofcoincidence counting information by the coincidence counting informationaccumulating section is terminated, the image reconstructing sectionreconstructs an image representing the spatial distribution ofoccurrence frequencies of photon pairs in the measurement space on thebasis of the accumulated coincidence counting information.

[0016] In the PET apparatus according to the above aspect of the presentinvention, in particular, the plurality of slice collimators arealternately arranged with the cylindrical detectors in a directionparallel to the central axis, and each slice collimator extends from aposition between the measurement space and the detecting section to therear portion of a corresponding one of the cylindrical detectors throughthe space between two adjacent cylindrical detectors. The respectiveslice collimators are then fixed to each other at the rear portions. Inthe PET apparatus according to the above aspect of the presentinvention, therefore, the precision of the relative positionalrelationship between the respective cylindrical detectors and therespective slice collimators is high, and the respective cylindricaldetectors and the respective slice collimators are always alternatelylocated in a direction parallel to the central axis. This ensures highincidence efficiency of photons on each cylindrical detector andsufficiently high performance. In addition, since there is no need tostrictly manage process accuracy and assembly accuracy for therespective cylindrical detectors, slice collimators, holding plate, andthe like, the apparatus can be easily manufactured at low cost.

[0017] If the holding plate for fixing the respective slice collimatorsto each other is placed at the rear portion of the detecting section,photons generated in the measurement space do not pass through theholding plate and are not absorbed by the holding plate. This prevents adecrease in the photon detection sensitivity of the detecting section.In addition, each slice collimator extends to the rear portion of acorresponding one of the cylindrical detectors through the space betweentwo adjacent cylindrical detectors and is fixed by the holding plate atthe rear portion. Photons passing through between two adjacent slicecollimators always strike the cylindrical detector between the two slicecollimators but do not strike the adjacent cylindrical detectors.Therefore, the reliability of accumulated coincidence countinginformation is high, and the quality of a reconstructed image isexcellent.

[0018] The PET apparatus according to the above aspect of the presentinvention is characterized in that the cylindrical detector is formed byarraying a plurality of two-dimensional position detectors, each ofwhich detects a two-dimensional position of a light-receiving surfacewhen a photon is incident thereon, on the predetermined plane in theform of a ring. In this case, this arrangement is suitable to improvethe resolution of a reconstructed image by reducing the size of eachphoton detection element. In this case, each cylindrical detector isdesigned such that a plurality of detector rings (each corresponding toone layer of photon detectors arrayed in the form of a ring in adirection parallel to the central axis) are stacked. Therefore,detection of coincidence counting information may be performed by a pairof photon detection elements in a single cylindrical detector includedin the detecting section, or a pair of photon detection elementsrespectively included in two adjacent cylindrical detectors depending onthe sizes of each cylindrical detector and each slice collimator, or apair of photon detection elements included in two separate cylindricaldetectors. In other words, detection of coincidence counting informationmay be performed between two adjacent detector rings or between twoseparate detector rings as well as within the single detector ring. Thatis, the PET apparatus according to the present invention has anintermediate arrangement between the two-dimensional PET apparatus shownin FIG. 7 and the three-dimensional PET apparatus shown in FIG. 8, andhas sensitivity about several times higher than that of the conventionaltwo-dimensional PET apparatus. The PET apparatus according to thepresent invention can therefore ensure good photon pair detectionsensitivity and quantitativeness while improving the resolution of areconstructed image.

[0019] A PET apparatus according to another aspect of the presentinvention comprises a plurality of photon detection elements whichdetect one photon and the other photon generated upon electron-positronpair annihilation with light-receiving surfaces facing a measurementspace, a plurality of block detectors which are formed bytwo-dimensionally arraying the plurality of photon detection elementsand arranged in a direction crossing a direction in which thelight-receiving surfaces face, and a plurality of collimators which arelocated between adjacent block detectors of the plurality of blockdetectors, extend from between the adjacent block detectors toward thelight-receiving surfaces, and guide only the photons flying from apredetermined direction to the plurality of photon detection elements,respectively.

[0020] According to the above aspect of the present invention, each ofthe plurality of collimators is placed between adjacent block detectorsof the plurality of block detectors. Therefore, since the precision ofthe relative positional relationship between the respective blockdetectors and the respective collimators is high, the photon detectionsensitivity can be improved.

[0021] According to the above aspect of the present invention, theapparatus further comprises holding means for holding the plurality ofcollimators, and the plurality of block detectors are located betweenthe measurement space and the holding means. According to thisarrangement, since photons generated in the measurement space are notabsorbed by the holding means, the photon detection sensitivity of theblock detectors does not deteriorate.

[0022] A PET apparatus according to still another aspect of the presentinvention is characterized by comprising (1) a detecting section whichincludes a plurality of cylindrical detectors each formed bytwo-dimensionally arraying a plurality of photon detection elements,each of which detects a photon flying from a measurement space includinga central axis, on a cylinder surrounding the central axis, theplurality of cylindrical detectors being arrayed in a direction parallelto the central axis, (2) a plurality of slice collimators which arealternately arranged with the cylindrical detectors at least between themeasurement space and the detecting section in a direction parallel tothe central axis, and pass only photons, of photons flying from themeasurement space, which are substantially parallel to a predeterminedplane perpendicular to the central axis toward the detecting section,(3) moving means for moving the detecting section and the plurality ofslice collimators together relative to an object to be examined which isplaced in the measurement space in a direction parallel to the centralaxis, (4) a coincidence counting information accumulating section whichacquires coincidence counting information of a photon pair detected byone pair of photon detection elements included in the detecting sectionduring a period in which the detecting section and the plurality ofslice collimators are moved relative to the object by the moving means,converts the coincidence counting information into information in acoordinate system fixed to the object, and accumulates the convertedinformation, and (5) an image reconstructing section which reconstructsan image representing a spatial distribution of occurrence frequenciesof photon pairs in the measurement space on the basis of the coincidencecounting information accumulated by the coincidence counting informationaccumulating section.

[0023] In the PET apparatus according to the above aspect of the presentinvention, the detecting section and the plurality of slice collimatorsare moved together relative to the object placed in the measurementspace by the moving means in a direction parallel to the central axis,and a photon pair, of photon pairs generated in the measurement space,which has reached the detecting section without being shielded by theslice collimator are simultaneously detected by one pair of photondetectors included in the detecting section. The resultant coincidencecounting information is converted into information in a coordinatesystem fixed to the object and accumulated by the coincidence countinginformation accumulating section. When accumulation of coincidencecounting information by the coincidence counting informationaccumulating section is completed, the image reconstructing sectionreconstructs an image representing the spatial distribution ofoccurrence frequencies of photon pairs in the measurement space on thebasis of the accumulated coincidence counting information.

[0024] In the PET apparatus according to the above aspect of the presentinvention, detection of coincidence counting information may beperformed by a pair of photon detectors in a single cylindrical detectorincluded in the detecting section, or a pair of photon detectorsrespectively included in two adjacent cylindrical detectors depending onthe sizes of each cylindrical detector and each slice collimator, or apair of photon detectors included in two separate cylindrical detectors.In other words, detection of coincidence counting information may beperformed between two adjacent detector rings or between two separatedetector rings as well as within the single detector ring (one layer ofphoton detectors arrayed in the form of a ring in a direction parallelto the central axis). That is, the PET apparatus according to stillanother aspect of the present invention has an intermediate arrangementbetween the two-dimensional PET apparatus shown in FIG. 7 and thethree-dimensional PET apparatus shown in FIG. 8, and has sensitivityabout several times higher than that of the conventional two-dimensionalPET apparatus. The PET apparatus according to still another aspect ofthe present invention can therefore ensure good photon pair detectionsensitivity and quantitativeness while improving the resolution of areconstructed image.

[0025] In the PET apparatus according to the above aspect of the presentinvention, coincidence counting information is accumulated by thecoincidence counting information accumulating section during a period inwhich the detecting section and slice collimators are moved togetherrelative to the object by the moving means in a direction parallel tothe central axis. A reconstructed image is then obtained by the imagereconstructing section on the basis of the accumulated coincidencecounting information. Even with the arrangement in which the cylindricaldetectors and slice collimators are alternately arranged, photon pairscan be detected with uniform sensitivity in the body axis direction ofthe object, and the quantitativeness of a reconstructed image can bemade uniform.

[0026] A PET apparatus according to still another aspect of the presentinvention comprises a plurality of photon detection elements whichdetect one photon and the other photon generated upon electron-positronpair annihilation with light-receiving surfaces facing a measurementspace, a plurality of block detectors which are formed bytwo-dimensionally arraying the plurality of photon detection elementsand arranged in a direction crossing a direction in which thelight-receiving surfaces face, a plurality of collimators which guideonly the photons flying from a predetermined direction to the pluralityof photon detection elements, respectively, a coincidence countinginformation accumulating section which accumulates, when one pair ofphoton detection elements included in the plurality of block detectorssimultaneously detect a photon pair, coincidence counting information ofthe photon pair detected by the one pair of photon detection elementsduring a period in which measurement is performed while the plurality ofblock detectors and the plurality of collimators are relatively movedtogether, and an image reconstructing section which reconstructs animage representing a spatial distribution of occurrence frequencies ofphoton pairs in the measurement space on the basis of the coincidencecounting information accumulated by the coincidence counting informationaccumulating section.

[0027] In the PET apparatus according to the above aspect of the presentinvention, coincidence counting information is accumulated by thecoincidence counting information accumulating section during a period inwhich the plurality of block detectors and the plurality of collimatorsare moved together relative to the object. A reconstructed image is thenobtained by the image reconstructing section on the basis of theaccumulated coincidence counting information. Photon pairs can bedetected with uniform sensitivity in the body axis direction of theobject, and the quantitativeness of a reconstructed image can be madeuniform.

BRIEF DESCRIPTION OF DRAWINGS

[0028]FIG. 1 is a schematic view showing the overall arrangement of aPET apparatus according to the embodiment;

[0029]FIG. 2 is a view for explaining the arrangement of the detectingsection and slice collimator of the PET apparatus according to thisembodiment;

[0030]FIG. 3 is an enlarged view partly showing the detecting sectionand slice collimator of the PET apparatus according to this embodiment;

[0031]FIG. 4 view for explaining the arrangement of cylindricaldetectors and slice collimators of the PET apparatus according to thisembodiment;

[0032]FIG. 5 is a view showing the arrangement of a block detectormounted in the PET apparatus according to this embodiment;

[0033]FIG. 6 is an enlarged view partly showing the detecting sectionand slice collimator of a one-dimensional PET apparatus according to amodification to this embodiment;

[0034]FIG. 7 is a view for explaining the arrangement of atwo-dimensional PET apparatus; and

[0035]FIG. 8 is a view for explaining the arrangement of athree-dimensional PET apparatus.

BEST MODES OF CARRYING OUT THE INVENTION

[0036] An embodiment of the present invention will be described indetail below with reference to the accompanying drawings. Note that thesame reference numerals denote the same elements throughout thedrawings, and a repetitive description will be avoided. It is an objectof this embodiment to provide an inexpensive PET apparatus which hashigh photon detection sensitivity, can accumulate highly reliablecoincidence counting information, and can be easily manufactured. It isanother object of the embodiment to provide a PET apparatus which canensure good photon pair detection sensitivity and quantitativeness whileimproving the resolution of a reconstructed image.

[0037]FIG. 1 is a view showing the schematic arrangement of the PETapparatus 1 according to this embodiment. FIG. 1 shows cross-sections ofa detecting section 10 and slice collimators 21 taken along a planeincluding a central axis CAX. The PET apparatus 1 according to theembodiment includes the detecting section 10, slice collimators 21 ₁, to21 ₁₁, a bed 31, a support base 32, a moving means 40, a coincidencecounting information accumulating section 50, an image reconstructingsection 60, and a display section 70. FIG. 1 also shows an object 2 tobe examined which is an examination target for the PET apparatus 1. Inaddition, a space where the coincidence counting information of photonpairs can be detected by the PET apparatus 1 is shown as a measurementspace 3.

[0038] The detecting section 10 has cylindrical detectors 13 ₁ to 13 ₁₂arranged in a direction parallel to the central axis CAX betweenring-like shield collimators 11 and 12. Each cylindrical detector 13_(n) is designed such that a plurality of photon detectors forrespectively detecting photons flying from the measurement space 3including the central axis CAX are two-dimensionally arranged on acylinder surrounding the central axis CAX. That is, each cylindricaldetector 13 _(n) is equivalent to a unit formed by stacking a pluralityof detector rings, each formed by arranging a plurality of photondetectors in form of a ring on a plane perpendicular to the central axisCAX, in a direction parallel to the central axis CAX. Each of the slicecollimators 21 ₁ to 21 ₁₁ is an example of a collimator. The slicecollimators 21 ₁ to 21 ₁₁ are alternately arranged on the cylindricaldetectors 13 ₁ to 13 ₁₂ in a direction parallel to the central axis CAXat least between the measurement space 3 and the detecting section 10 topass only photons, of the photons flying from the measurement space 3,which are substantially parallel to a predetermined plane toward thedetecting section 10. The detecting section 10 and slice collimators 21₁ to 21 ₁₁ will be described in detail later.

[0039] The bed 31 is used to place the object 2 thereon, and supportedby the support base 32. The moving means 40 moves the detecting section10 and slice collimators 21 ₁ to 21 ₁₁ together relative to the object 2placed in the measurement space 3 in a direction parallel to the centralaxis CAX. This movement is an example of the operation of relativelymoving the plurality of block detectors and the plurality of collimatorstogether in a direction crossing the direction in which thelight-receiving surfaces face. More specifically, the moving means 40may move the detecting section 10 and slice collimators 21 ₁ to 21 ₁₁together in a direction parallel to the central axis CAX or may move thebed 31 (i.e., the object 2) in a direction parallel to the central axisCAX. In addition, the moving means 40 may move them in only onedirection during measurement or reciprocate them. Relative movement isdone by the moving means 40 such that the region of interest of theobject 2 is moved by a distance equal to or more than ½ the arrangementpitch of the respective cylindrical detectors 13 _(n) duringmeasurement. Preferably, the region of interest of the object 2 isrelatively moved at a constant speed in the measurement space 3 by adistance corresponding to an integer multiple of the above pitch duringmeasurement. If the regions of interest of the object 2 exist over apredetermined range in the central axis CAX direction; it is preferablethat each region in the predetermined range stay in the measurementspace 3 for an almost constant period of time during measurement.

[0040] The coincidence counting information accumulating section 50accumulates coincidence counting information of photon pairs detected byone pair of photon detectors included in the detecting section 10 duringa period in which the detecting section 10 and slice collimators 21 ₁ to21 ₁₁ are moved relative to the object 2 by the moving means 40. Inaccumulating coincidence counting information, the coincidence countinginformation detected by the detecting section 10 is converted intoinformation in a coordinate system fixed to the object 2 on the basis ofthe displacement amounts of the slice collimators 21 ₁ to 21 ₁₁, anddetecting section 10 relative to the object 2, and the coincidencecounting information having undergone this coordinate conversion isaccumulated. Note that as the relative displacement amounts, the dataobtained by an encoder or the like or the data recorded by the movingmeans 40 may be used.

[0041] The image reconstructing section 60 reconstructs an imagerepresenting the spatial distribution of occurrence frequencies ofphoton pairs in the measurement space 3 on the basis of the coincidencecounting information accumulated by the coincidence counting informationaccumulating section 50. The image reconstructing section 60 alsoperforms sensitivity correction to correct variations in detectionsensitivity of the respective photon detectors of the detecting section10, absorption correction to correct the absorption of photons in theobject 2, and scatter correction to correct scattering of photons in theobject 2. The display section 70 displays the reconstructed imageobtained by the image reconstructing section 60. A control section 80controls relative movement done by the moving means 40, accumulation ofcoincidence counting information by the coincidence counting informationaccumulating section 50, image reconstruction done by the imagereconstructing section 60, and display of a reconstructed image by thedisplay section 70.

[0042]FIG. 2 is a view for explaining the arrangements of the detectingsection 10 and slice collimator 21 of the PET apparatus according tothis embodiment. FIG. 2 shows cross-sections of the detecting section 10and slice collimators 21 ₁ to 21 ₁₁ taken along a plane including thecentral axis CAX. The detecting section 10 of the PET apparatusaccording to this embodiment includes the cylindrical detectors 13 ₁ to13 ₁₂ stacked in a direction parallel to the central axis CAX betweenthe ring-like shield collimators 11 and 12. The respective ring-likeslice collimators 21 ₁ to 21 ₁₁ are alternately arranged with thecylindrical detectors 13 _(n) in a direction parallel to the centralaxis CAX. That is, each slice collimator 21 _(n) is placed between thecylindrical detector 13 _(n) and a cylindrical detector 13 _(n+1) whichare adjacent to each other. Each slice collimator 21 _(n) is made of amaterial having a larger atomic number and larger specific gravity(e.g., lead or tungsten) and several mm (e.g., 5 mm to 6 mm) thick. Eachslice collimator 21 _(n) has a collimating function of passing photons,of photons flying from the measurement space, which are substantiallyparallel to a plane perpendicular to the central axis CAX toward thedetecting section 10 and shielding obliquely incident photons.

[0043]FIG. 3 is an enlarged view of a portion (the portion enclosed withthe chain line in FIG. 2) of the detecting section 10 and slicecollimators 21 of the PET apparatus according to this embodiment. FIG. 4is a view for explaining the arrangement of the cylindrical detector 13_(n) and slice collimator 21 _(n) of the PET apparatus according to thisembodiment. FIG. 4 shows the relationship between the cylindricaldetector 13 _(n) and the slice collimator 21 _(n) when viewed from adirection parallel to the central axis CAX. Each cylindrical detector 13_(n) has a plurality of block detectors 41 ₁ to 14 _(M) arranged in theform of a ring on the same circumference on a plane perpendicular to thecentral axis CAX. Each block detector 14 _(m) serves as atwo-dimensional position detector for detecting the two-dimensionalincident position of a photon incident on an the light-receiving surface15 b. Each slice collimator 21 _(n) reaches the rear portion of thecorresponding cylindrical detector 13 _(n) through the space between thecylindrical detector 13 _(n) and the cylindrical detector 13 _(n+1)which are adjacent to each other and is integrally fixed to a holdingplate 22 at the rear portion. The holding plate 22 is an example of aholding means. As shown in FIG. 2, the plurality of block detectors 14provided for the respective cylindrical detectors 13 _(n) are locatedbetween the measurement space 3 and the holding plate 22. As shown inFIG. 3, the respective slice collimators 21 _(n) are located between theadjacent block detectors 14, extend from between the adjacent blockdetectors 14 toward the light-receiving surfaces 15 b, and guide onlyphotons flying from a predetermined direction toward a plurality ofphoton detection elements 15 a. Each slice collimator 21 _(n) extendsfrom between the adjacent block detectors 14 toward the holding plate22. As shown in FIGS. 3 and 4, the plurality of block detectors 41 ₁ to14 _(M) provided for the respective cylindrical detectors 13 _(n) are soarranged as to form a cylindrical shape surrounding the measurementspace 3.

[0044]FIG. 5 is a view showing the arrangement of a block detector 14.As shown in FIG. 5, each block detector 14 _(m) is a scintillationdetector formed from a combination of a scintillation block 15constituted by P×Q (P≧2, Q≧2) segments, and a position detection typephotomultiplier 16. Each block detector 14 _(m) detects a photon flyingfrom the measurement space and also detects the two-dimensional incidentposition of the photon incident on the light-receiving surface 15 b ofthe scintillation block 15. That is, the block detector 14 _(m) isequivalent to a unit obtained by two-dimensionally arranging P×Q smallphoton detectors 15 a. Each cylindrical detector 13 _(n) constituted bysuch block detectors 14 _(m) arranged in the form of a ring isequivalent to a unit obtained by stacking a plurality of detector rings,each constituted by a plurality of photon detectors 15 a arranged in theform of a ring on a plane perpendicular to the central axis CAX, in adirection parallel to the central axis CAX. In the block detector 14_(m), a resistor array for applying a predetermined voltage to eachelectrode in the position detection type photomultiplier 16 and apreamplifier for receiving the current signal output from the anodeelectrode of the position detection type photomultiplier 16 andoutputting it as a voltage signal are housed in a casing, together withthe scintillation block 15 and position detection type photomultiplier16, for light shielding.

[0045] For example, BGO (Bi₄Ge₃O₁₂), GSO (Gd₂SiO₅(Ce)), LSO(Lu₂SiO₅(Ce)), or PWO (PbWO₄) is used for the scintillation block 15, asneeded. The scintillation block 15 is constituted by 8×8 segments, andeach segment has a size of 6 mm×6 mm×20 mm. The area of thephotoelectric surface of the position detection type photomultiplier 16is 50 mm×50 mm. The cylindrical detector 13 _(n) is formed by arranging60 block detectors 14 _(m), each including the scintillation block 15and position detection type photomultiplier 16, in the form of a ring.Each cylindrical detector 13 _(n) has an inner diameter of about 1,000mm. Each slice collimator 21 _(n) has an inner diameter of 600 mm. Thestructure formed by alternately stacking the cylindrical detectors 13 ₁to 13 ₁₂ and slice collimators 21 ₁ to 21 ₁₁ in a direction parallel tothe central axis CAX has a thickness (i.e., the visual field in the bodyaxis direction) of about 670 mm.

[0046] Under such conditions, detection of a pair of 511 keV photons(gamma rays) generated upon electron-positron pair annihilation in themeasurement space 3 and flying in opposite directions, i.e., detectionof coincidence counting information, may be performed by a pair of blockdetectors 14 in the same cylindrical detector 13 _(n) or a pair of blockdetectors 14 respectively included in the adjacent cylindrical detectors13 _(n) and 13 _(n+1). Detection of coincidence counting information maybe performed by a pair of block detectors 14 included in two separatecylindrical detectors 13. In other words, detection of coincidencecounting information may be performed between two adjacent detectorrings or two separate detector rings as well as within the singledetector ring.

[0047] That is, the PET apparatus 1 according to this embodiment has anintermediate arrangement between the two-dimensional PET apparatus shownin FIG. 7 and the three-dimensional PET apparatus. shown in FIG. 8 Whensensitivity per unit visual field length (cm) in the body axis directionunder the above conditions is calculated, the sensitivity of the PETapparatus 1 according to this embodiment is about 1.3 kcps/(kBq·ml),which is about ½ the sensitivity (about 2.58 kcps/(kBq·ml)) of theconventional three-dimensional PET apparatus, but is about four to fivetimes higher than the sensitivity (about 0.28 kcps/(kBq·ml)) of theconventional two-dimensional PET apparatus.

[0048] The operation of the PET apparatus 1 according to this embodimentwill be described next. The object 2 to which an RI is applied is placedon the bed 31, and the region of interest of the object 2 is positionedin the measurement space 3. The PET apparatus 1 operates in thefollowing manner under the control of the control section 80. First ofall, the moving means 40 starts to move the detecting section 10 andslice collimators 21 ₁ to 21 ₁₁ together relative to the object 2 placedin the measurement space 3 in a direction parallel to the central axisCAX.

[0049] Of the 511 keV photons (gamma rays) generated uponelectron-positron pair annihilation in the measurement space 3, photonsthat have reached the detecting section 10 without being shielded by theslice collimators 21 ₁₁ to 21 ₁₁ are simultaneously detected by one pairof photon detection elements 15 a included in the detecting section 10.The coincidence counting information simultaneously detected by thedetecting section 10 during the period of this relative movement isconverted into information in a coordinate system fixed to the object 2on the basis of the relative displacement amounts of the detectingsection 10 and slice collimators 21 ₁ to 21 ₁₁ with respect to theobject 2. The resultant information is accumulated by the coincidencecounting information accumulating section 50.

[0050] When a predetermined measurement period comes to an end, thecoincidence counting information accumulating section 50 stopsaccumulating coincidence counting information, and the moving means 40also stops relative movement. The image reconstructing section 60reconstructs an image representing the spatial distribution ofoccurrence frequencies of photon pairs in the measurement space 3 on thebasis of the coincidence counting information accumulated by thecoincidence counting information accumulating section 50. The imagereconstructing section 60 also performs sensitivity correction,absorption correction, and scatter correction. The reconstructed imageobtained by the image reconstructing section 60 is displayed by thedisplay section 70.

[0051] As described above, in the PET apparatus 1 according to thisembodiment, the slice collimator 21 _(n) reaches the rear portion ofeach cylindrical detector 13 _(n) through the space between thecylindrical detector 13 _(n) and the cylindrical detector 13 _(n+1), andis integrally fixed by the holding plate 22 at the rear portion. In thePET apparatus 1 according to this embodiment, therefore, the precisionof relative positional relationship between each cylindrical detector 13_(n) and a corresponding one of the slice collimators 21 _(n) is high,and the-respective cylindrical detectors 13 _(n) and the respectiveslice collimators 21 _(n) are alternately arranged in a directionparallel to the central axis CAX. This ensures high incidence efficiencyof photons on each cylindrical detector 13 _(n) and sufficiently highperformance. In addition, since there is no need to strictly manageprocess accuracy and assembly accuracy for the respective cylindricaldetectors 13 _(n), slice collimators 21 _(n), holding plate 22, and thelike, the apparatus can be easily manufactured at low cost. Furthermore,this arrangement is suitable to move the detecting section 10 and slicecollimators 21 ₁ to 21 ₁₁ together in a direction parallel to thecentral axis CAX.

[0052] In the PET apparatus 1 according to this embodiment, since theholding plate 22 for fixing the slice collimators 21 _(n) to each otheris located at the rear portion of the detecting section 10, photonsgenerated in the measurement space 3 do not pass through the holdingplate 22 and are not absorbed by the holding plate 22. Therefore, thephoton detection sensitivity of the detecting section 10 does notdecrease.

[0053] In addition, in the PET apparatus 1 according to this embodiment,the slice collimator 21 _(n) extends to the rear portion of eachcylindrical detector 13 _(n) through the space between the cylindricaldetector 13 _(n) and the cylindrical detector 13 _(n+1) and is fixed bythe holding plate 22 at the rear portion, photons passing throughbetween the slice collimator S_(n−1) and the slice collimator S_(n)always strike the cylindrical detector 13 _(n) but do not strike theadjacent cylindrical detector 13 _(n−1) or cylindrical detector 13_(n+1). Therefore, the reliability of accumulated coincidence countinginformation is high, and the quality of a reconstructed image is high.

[0054] The detecting section 10 of the PET apparatus 1 according to thisembodiment has the cylindrical detectors 13 ₁ to 13 ₁₂ arranged in adirection parallel to the central axis CAX. Each cylindrical detector 13_(n) is designed such that a plurality of photon detection elements 15 aare two-dimensionally arranged on a cylinder surrounding the centralaxis CAX. The cylindrical detectors 13 ₁ to 13 ₁₂ and slice collimators21 ₁ to 21 ₁₁ are alternately arranged in a direction parallel to thecentral axis CAX. This arrangement of the detecting section 10 makes itpossible to improve the resolution of a reconstructed image by reducingthe size of each photon detection element 15 a. In addition, since theslice collimators 12 _(n) are not arranged between the detector ringsbut are arranged between the cylindrical detectors 13 _(n), intervalsare ensured between the respective slice collimators 12 _(n) to suppressa decrease in open area ratio, thus ensuring high photon pair detectionsensitivity. Furthermore, since each slice collimator 12 _(n) need notbe thinned, the collimating effect can be maintained, and scatteredradiation can be efficiently removed. This makes it possible to ensurehigh quantitativeness of a reconstructed image. As described above, thePET apparatus 1 according to this embodiment can ensure good photon pairdetection sensitivity and quantitativeness while improving theresolution of a reconstructed image.

[0055] While the detecting section 10 and slice collimators 21 ₁ to 21₁₁ are moved together relative to the object 2 in a direction parallelto the central axis CAX by the moving means 40, coincidence countinginformation is accumulated by the coincidence counting informationaccumulating section 50, and a reconstructed image is obtained by theimage reconstructing section 60 on the basis of this accumulatedcoincidence counting information. In this embodiment, therefore, evenwith the above arrangement of the cylindrical detectors 13 ₁ to 13 ₁₂and slice collimators 21 ₁ to 21 ₁₁, photon pairs can be detected withuniform sensitivity in the body axis direction of the object 2, and thequantitativeness of a reconstructed image can be made uniform.

[0056] In addition, in this embodiment, each cylindrical detector 13_(n) is formed from a ring-like array of a plurality of two-dimensionaldetectors (block detectors 14 _(m)) which detect the two-dimensionalincident positions of photons incident on the light-receiving surfaces15 b. This arrangement is therefore suitable to improve the resolutionof a reconstructed image by reducing the size of each photon detectionelement 15 a.

[0057] The present invention is not limited to the above embodiment andcan be variously modified. For example, in the above embodiment, thecylindrical detector 13 _(n) located between the adjacent slicecollimators S_(n−1) and S_(n) is designed such that a plurality ofphoton detection elements 15 a are two-dimensionally arrayed on acylinder surrounding the central axis CAX, and a plurality of detectorrings (each corresponding to one layer of photon detection elements 15 aarrayed in the form of a ring in a direction parallel to the centralaxis) are stacked in a direction parallel to the central axis CAX.However, as shown in FIG. 6, each cylindrical detector 13 _(n) may bedesigned such that a plurality of photon detection elements 15 a areone-dimensionally arrayed on a cylinder surrounding the central axis CAX(i.e., one layer of detector ring). The same reference numerals as inFIG. 3 denote the same elements in FIG. 6, and a detailed descriptionthereof will be omitted.

[0058] Industrial Applicability

[0059] As has been described in detail above, according to the PETapparatus of the present invention, the photon detection sensitivity canbe increased.

1. (Amended) A pet apparatus comprising: a detecting section whichincludes a plurality of cylindrical detectors each formed bytwo-dimensionally arraying a plurality of photon detection elements,each of which detects a photon flying from a measurement space includinga central axis, on a cylinder surrounding the central axis, theplurality of cylindrical detectors being arrayed in a direction parallelto the central axis; a plurality of slice collimators which arealternately arranged with said cylindrical detectors in a directionparallel to the central axis, each of said slice collimators extendingfrom a position between the measurement space and said detecting sectionto a rear portion of a corresponding one of the cylindrical detectorsthrough a space between two adjacent cylindrical detectors of theplurality of cylindrical detectors and passing only a photon, of photonsflying from the measurement space, which is substantially parallel to apredetermined plane perpendicular to the central axis toward saiddetecting section; a coincidence counting information accumulatingsection which accumulates coincidence counting information of a photonpair detected by one pair of photon detection elements included in saiddetecting section; and an image reconstructing section whichreconstructs an image representing a spatial distribution of occurrencefrequencies of photon pairs in the measurement space on the basis of thecoincidence counting information accumulated by said coincidencecounting information accumulating section.
 2. A PET apparatus accordingto claim 1, wherein said cylindrical detector is formed by arraying aplurality of two-dimensional position detectors, each of which detects atwo-dimensional position of a light-receiving surface when a photon isincident thereon, on the predetermined plane in the form of a ring. 3.(Amended ) A PET apparatus comprising: a plurality of photon detectionelements which detect one photon and the other photon generated uponelectron-positron pair annihilation with light-receiving surfaces facinga measurement space; a plurality of block detectors which are formed bytwo-dimensionally arraying said plurality of photon detection elementsand arranged in a direction crossing a direction in which thelight-receiving surfaces face; and a plurality of collimators which arelocated between the respective adjacent block detectors of saidplurality of block detectors, extend from between the adjacent blockdetectors toward the light-receiving surfaces, and guide only thephotons flying from a predetermined direction to said plurality ofphoton detection elements, respectively.
 4. A PET apparatus according toclaim 3, wherein said apparatus further comprises holding means forholding said plurality of collimators, and said plurality of blockdetectors are located between the measurement space and said holdingmeans.
 5. A PET apparatus according to claim 3 or 4, wherein saidplurality of collimators extend from between adjacent block detectors ofsaid plurality of block detectors toward said holding means.
 6. A PETapparatus according to any one of claims 3 to 5, wherein said pluralityof block detectors are so arranged as to form a cylindrical shapesurrounding the measurement space.
 7. A PET apparatus comprising: adetecting section which includes a plurality of cylindrical detectorseach formed by two-dimensionally arraying a plurality of photondetection elements, each of which detects a photon flying from ameasurement space including a central axis, on a cylinder surroundingthe central axis, the plurality of cylindrical detectors being arrayedin a direction parallel to the central axis; a plurality of slicecollimators which are alternately arranged with said cylindricaldetectors at least between the measurement space and said detectingsection in a direction parallel to the central axis, and pass onlyphotons, of photons flying from the measurement space, which aresubstantially parallel to a predetermined plane perpendicular to thecentral axis toward said detecting section; moving means for moving saiddetecting section and said plurality of slice collimators togetherrelative to an object to be examined which is placed in the measurementspace in a direction parallel to the central axis; a coincidencecounting information accumulating section which acquires coincidencecounting information of a photon pair detected by one pair of photondetection elements included in said detecting section during a period inwhich said detecting section and said plurality of slice collimators aremoved relative to the object by said moving means, converts thecoincidence counting information into information in a coordinate systemfixed to the object, and accumulates the converted information; and animage reconstructing section which reconstructs an image representing aspatial distribution of occurrence frequencies of photon pairs in themeasurement space on the basis of the coincidence counting informationaccumulated by said coincidence counting information accumulatingsection.
 8. A PET apparatus according to claim 7, wherein saidcylindrical detector is formed by arraying a plurality oftwo-dimensional position detectors, each of which detects atwo-dimensional position of a light-receiving surface when a photon isincident thereon, on the predetermined plane in the form of a ring.
 9. APET apparatus according to claim 7 or 8, wherein each of said pluralityof slice collimators extends to a rear portion of a corresponding one ofsaid cylindrical detectors through a space between two adjacentcylindrical detector of said plurality of cylindrical detectors includedin said detecting section.
 10. A PET apparatus according to any one ofclaims 7 to 9, wherein said detecting section and said plurality ofslice collimators are moved together in a direction parallel to thecentral axis by said moving means.
 11. A PET apparatus according to anyone of claims 7 to 9, wherein said apparatus further comprises a bedplaced in the measurement space, on which an object to be examined isplaced, and said bed is moved in a direction parallel to the centralaxis by said moving means.
 12. A PET apparatus according to any one ofclaims 7 to 11, wherein said detecting section and said plurality ofslice collimators are moved together relative to the object placed inthe measurement space in a direction parallel to the central axis bysaid moving means.
 13. A PET apparatus according to any one of claims 7to 11, wherein said detecting section and said plurality of slicecollimators are reciprocally moved together relative to the objectplaced in the measurement space in a direction parallel to the centralaxis by said moving means.
 14. A PET apparatus according to any one ofclaims 7 to 13, wherein the relative movement is performed within themeasurement space by a distance corresponding to not less than ½ anarrangement pitch of said plurality of cylindrical detectors in theperiod.
 15. A PET apparatus according to claim 14, wherein the distanceis an integer multiple of the pitch.
 16. A PET apparatus according toany one of claims 7 to 15, wherein the relative movement is performed ata constant speed.
 17. A PET apparatus according to any one of claims 7to 16, wherein when regions of interest of the object exist over apredetermined range in the central axis direction, each region in thepredetermined range stays in the measurement space for a substantiallyconstant period of time during the period.
 18. (Amended) A PET apparatuscomprising: a plurality of photon detection elements which detect onephoton and the other photon generated upon electron-positron pairannihilation with light-receiving surfaces facing a measurement space; aplurality of block detectors which are formed by two-dimensionallyarraying said plurality of photon detection elements and arranged in adirection crossing a direction in which the light-receiving surfacesface; a plurality of collimators which are arranged between therespective adjacent block detectors of said plurality of block detectorsguide only the photons flying from a predetermined direction to saidplurality of photon detection elements, respectively; a coincidencecounting information accumulating section which accumulates, when onepair of photon detection elements included in said plurality of blockdetectors simultaneously detect a photon pair, coincidence countinginformation of the photon pair detected by said one pair of photondetection elements during a period in which measurement is performedwhile said plurality of block detectors and said plurality ofcollimators are relatively moved together; and an image reconstructingsection which reconstructs an image representing a spatial distributionof occurrence frequencies of photon pairs in the measurement space onthe basis of the coincidence counting information accumulated by saidcoincidence counting information accumulating section.
 19. A PETapparatus according to claim 18, further comprising moving means forrelatively moving said plurality of block detectors and said pluralityof collimators together in a direction crossing a direction in which thelight-receiving surfaces face.
 20. A PET apparatus according to claim19, wherein said plurality of block detectors and said plurality ofcollimators are moved together in a direction crossing the direction inwhich the light-receiving surfaces face.
 21. A PET apparatus accordingto claim 19, wherein said apparatus further comprises a bed placed inthe measurement space, and said bed is moved by said moving means in adirection crossing the direction in which the light-receiving surfacesface.